1. Field of the Invention
The present invention deals with the field of seals for rotary machines and to a process for producing leaves for a leaf seal.
2. Brief Description of the Related Art
A gas turbine includes a rotor, on which there are arranged various stages having compressor blades and turbine blades, as well as a stator housing. The rotor is mounted in bearings at each end of the rotor shaft.
Control of the gas flow within the gas turbine is of great importance both with regard to the functionality and with regard to the efficiency. At various points along the rotor shaft, seal technologies are used to reduce the axial flow of gas along the shaft. This is particularly important next to the bearings, in order to prevent the oil used in the bearings from being overheated by the hot gases of the gas stream.
Traditionally, two types of seal technologies—generally as alternatives but sometimes also in combination with one another—are used in this situation. These are labyrinth seals and brush seals.
In labyrinth seals, there is no metal-to-metal contact between the rotor and the stator; therefore, their sealing effect is relatively slight. However, they offer the advantage of low rotary friction, and consequently a virtually unlimited service life.
Brush seals, on the other hand, have higher friction losses on account of the friction between the ends of the bristles and the rotor shaft. This leads to wear, which limits the service life of the seal. However, brush seals offer improved inhibition of the axial gas flow, particularly in the case of relatively high axial pressure differences.
There are numerous restrictions on the use of these technologies for sealing purposes in gas turbines. First, the axial pressure difference which they are able to withstand is still rather low. In the case of brush seals, this is because of the bristles, which have the same rigidity in the axial and circumferential directions: high pressures can cause the bristles to blow back onto themselves in the axial direction. The seals also have little ability to permit and withstand significant radial movement.
The design of a brush seal is often a compromise between the use of a support plate, which is intended to provide sufficient axial support, and the aim not to restrict the radial movement.
To avoid the drawbacks of the known brush seals, U.S. Pat. No. B1 6,343,792 has proposed a leaf seal which performs the same function as either a labyrinth seal or a brush seal but has the advantages of both. Instead of the bristles, which are made from wires of circular cross section, thin metal leaves or lamellae are assembled in a set arrangement (cf. for example FIG. 3 of U.S. Pat. No. B1 6,343,792 or FIG. 1 of the present application). The leaves, whose surfaces are oriented substantially parallel to the axial direction, are much more rigid in the axial direction than in the circumferential direction. This means that the seal is able to withstand higher pressure differences without its ability to permit radial movements being restricted. Also, the wide area on the rotor which is covered by the tips of the leaves offers the option of generating a hydrodynamic force which can separate the leaf tips from the shaft during operation. This makes it possible to produce and maintain a spacing of a few micrometers, so that the wear, frictional heat and friction losses are reduced to almost zero.
The basic design incorporates a number of thin metal leaves which between them have a controlled air gap and are secured at a predetermined angle with respect to the radius. The air gap is a critical design parameter. It allows an air flow to occur in order thereby to generate the hydrodynamic effect; however, it must not be large enough to permit an excessive axial leakage flow.
Two variants of the leaf seal design are possible: in one variant, the leaves are blown downward, while in the other variant they are blown upward. The variant with the leaves which are blown downward includes the possibility of there being a spacing between the leaf tips and the shaft during assembly and start-up and of this gap being decreased in size to very small values by the use of an air stream between the leaves. On the other hand, the variant in which the leaves are blown upward includes the possibility of the leaf tips and shaft influencing one another slightly during start-up, and of producing a spacing when the shaft accelerates. In both cases, the flow of the medium through the air gaps between the leaves is critical, as is the control of the internal diameter of the seal which is produced by the leaf tips.
The air flow through the leaves can be altered by using a front plate and a rear plate, which leave clear a narrow gap between the surfaces of the set of leaves and the plates (cf. the above-referenced FIG. 1 and 3). Careful design of these geometries makes it possible to control the upward or downward blowing effects. It may also be desirable for the downward blowing effect to be assisted by active supply of pressure along the length of the leaves or inward from the front side or from the rear-side directions.
One of the other main advantages of the leaf seal concept is a greater tolerance for the radial movement than in the case of the labyrinth or brush seals. There, this requires a considerable distance between the internal diameter of the front and rear-side plates and the shaft.
Depending on the selected geometry for the seal and on the diameter of the shaft to be sealed, the number of leaves may be a few thousand or tens of thousands. The accuracy with which they can be produced, assembled, and connected, while ensuring a reproducible air gap between each pair of leaves, is critical for successful implementation of any possible seal design.
The joining method for fixing the leaves in position could be a mechanical technique, such as clamping, welding, or brazing, or any possible combination thereof. In this context, it is very obviously important that there should be minimum disruption to the leaves or their relative positions during the joining process.
In particular the spacer elements which are placed between the leaves and set the width of the air gap are of importance in this context. The spacer elements can be designed as separate elements. This has the advantage that the leaves are relatively simple to produce, as elements of constant thickness. However, this also has the drawback that the spacer elements have to be produced separately and that the alternating assembly of the leaves and spacer elements to form a set of leaves is complex and can easily give rise to positioning errors. However, it is also possible for the spacer elements to be designed as integral elements of the leaves, which considerably simplifies assembly. In this case, however, the leaves have to be subjected to special machining in order to form the regions of different thicknesses.
To this end, it has already been proposed in the above-referenced U.S. Pat. No. B1 6,343,792 (cf. FIG. 27B of that document and the associated description in column 21, lines 25-34) that the stepped thickness regions of the leaves be produced with an integral spacer element by means of etching. The etching technique allows accurate setting of the thicknesses. However, a drawback in this case is that the etching, after the leaves have been cut to size, introduces an additional process step which complicates and lengthens production.